1. Field of the Invention
This invention relates to a thermostable tryptophan synthetase gene originating in extremely thermophilic microorganism and an extremely thermophilic microorganism plasmid vector incorporating the gene as a marker therein.
The tryptophan synthetase is an enzyme which synthesizes tryptophan, an essential amino acid, from indole and serine. The gene of this enzyme can be used for inexpensive production of tryptophan. The plasmid vector can be used for breeding an extremely thermophilic microorganism by genetic manipulation.
2. Prior Art Statement
Extremely thermophilic microorganisms capable of growing at high temperatures over 75.degree. C. produce biopolymers which are stable against of heat and exhibit high tolerance to chemical reagents and organic solvents. Therefore, studies have been made on use of the microorganisms in fermentative production at elevated temperatures and on bioreactors by immobilizing the microorganisms or their enzymes. Efficient breeding of such extremely thermophilic microorganisms is an essential requirement for such purposes. While recombinant DNA technique is the most promising means of breeding at present, this method cannot be used with extremely thermophilic microorganisms because they have no antibiotic-resistant plasmid, e.g., they have no plasmid vector.
The inventors have already developed a method for transformation, i.e. a technique for the introduction of a gene DNA, with respect to Thermus thermophilus HB 27, a typical species of gram-negative extremely thermophilic microorganisms [Japanese Patent Public Disclosure SHO 60(1985)-188076]. They have also succeeded in separating the 3.1 Kb DNA segment including a tryptophan synthetase gene [Japanese Patent Application SHO 62(1987)-297129]and have studied the feasibility of coupling the DNA segment mentioned above with the cryptic plasmid of a microorganism of genus Thermus and utilizing the product of the coupling as a selective marker gene. They have failed to produce a vector plasmid, however, because the DNA segment in the strain of Thermus thermophilus HB 27 as the host frequently undergoes recombination with the chromosome DNA and eventual incorporation in the chromosome DNA and cannot be used as a selective marker.